Istanbul, Turkey — The scale of Iran's suicide bombing on Wednesday – in which at least 39 Iranian Shiite worshipers were killed and 50 injured during the penultimate day of a important religious commemoration – may have surprised Iranian security officials in the southern port city of Chabahar.

Why? Because Iranian officials have repeatedly declared victory over the group that claimed responsibility, Jundallah (Soldiers of God).

In February, Iran captured Abdolmalek Rigi, of the shadowy Sunni and ethnic Baluch militant group active in southeast Iran and executed him in June. Jundallah retaliated with today's attack, the group said in a statement accompanied by photos of the alleged attackers.

“This operation was a revenge for the hanging of the head of the movement Abdolmalek and other members of Jundallah,” the statement read. “In this suicide operation…tens of [Revolutionary] Guards and mercenaries have been killed. The operation was carried out to expose the aggressors of Baluchistan.”

Iranian officials claimed that security forces spotted the two bombers, and shot one before he could detonate his explosives. They said they arrested a third man, the “ringleader,” and alleged that foreign spy agencies were behind the carnage.

Ali Abdollahi, Iran’s deputy interior minister, said the “equipment used shows that they are terrorists supported by the intelligence services of the region and the US,” in remarks reported by the state news agency IRNA.

Iran’s state-run PressTV said it had “learned” that explosives had been provided by Saudi Arabia – whose king was reported to have asked the United States to bomb Iran to “cut off the head of the snake,” according to US diplomatic cables released by WikiLeaks.

Alaeddin Borujerdi, the head of the parliament’s foreign affairs committee, said the “intelligence services of the United States and Britain” were behind the attack, according to the ISNA news agency.

President Barack Obama spoke out strongly against the attack, however, which fell during the Shiite Muslim holiday of Ashura.

"I strongly condemn the outrageous terrorist attack on a mosque in Chabahar, Iran," said Obama in a written statement. "The murder of innocent civilians in their place of worship during Ashura is a despicable offense, and those who carried it out must be held accountable. This is a disgraceful and cowardly act."

Attackers struck sooner than expected

Three photographs accompanied the Jundallah statement. The first image shows the two young men – one appears young enough to still be a school student – posing with AK-47 assault rifles and with two plastic-wrapped suicide vests in front of them.

A second image shows the younger attacker standing alone with a hint of a grin on his boyish face, and wearing his suicide vest over a white T-shirt, the package of explosives held in place by two colorful shoulder straps.

The third image shows the smiling, older attacker with longish hair, a baseball cap and a trace of beard, with hands in the front pocket of his jeans – to all appearance a typical Iranian youth, except for the wired explosive vest around his waist.

State television reported that for the past three days Iranian authorities had received threats of imminent Jundallah attacks in Iran’s remote Sistan-Baluchistan Province, which abuts the border with Pakistan and, further north, Afghanistan.

But security would have been tightened most in the provincial capital of Zahedan – the site of many previous attacks – and for the actual peak of Ashura on Thursday. That day marks the 7th-century death of Hossein, the revered third imam for Shiites, at the hands of a Sunni army.

“Their goal is to divide the Shiites and Sunnis, and to incite differences between them,” said a mourner named Rahimi after the blasts, according to news agency footage that aired on Al Jazeera English. “In the view of all freedom-seekers, it is not a good action and is condemned.”

Iran has long accused US of backing Jundallah

Despite the execution of Jundallah’s leader – who was captured when his commercial flight from Dubai to Kyrgyzstan was forced to land by Iranian fighter planes last February – Jundallah has vowed to keep up what they consider a fight for more rights for ethnic Baluchs in Iran.

Jundallah has for years targeted both civilians and officers of Iran’s elite Revolutionary Guard unit, including high-profile bomb attacks such as the one that killed 42 people in October 2009 – among them seven ranking Guard officers and senior tribal leaders. Other attacks killed 20 worshipers in a mosque the previous May; and 11 Guard soldiers died when their bus was attacked in February 2004.

Tehran has long accused Washington of supporting Jundallah “terrorists” against the Islamic Republic, in a secret policy of backing minority groups with grievances against the regime. Iran’s top intelligence officials in February showed what they claimed were photographs of Rigi just days before his capture, on a US military base in Afghanistan.

In a televised “confession” after his arrest, Rigi himself said Jundallah had received US support, echoing earlier claims made by Rigi’s brother of US help. In 2007, ABC News quoted US officials saying anonymously that American cash was funneled through non-US intermediaries.

After Rigi’s arrest the regional governor had said: “The arrest has returned complete security to the region, and this province will follow the path of progress rapidly.”

And yet the desert region was struck by another bomb attack in July, in which Jundallah claimed responsibility for the deaths of 28, including Revolutionary Guard officers, at the Grand Mosque in Zahedan.

Despite the number of lethal attacks going back a decade, Jundallah was only added last month to the US State Department’s global list of terrorist groups.